1. Field of the Invention
The present invention relates to electric circuit designs. Particularly, the present invention relates to complimentary single-ended-input OTA-C universal filter structures. “OTA-C” stands for “operational transconductance amplifier and capacitor.”
2. Description of the Prior Art
Voltage or current-mode nth-order OTA-C filter structures have been investigated and developed for several years. Recently, the analytical synthesis methods (ASMs) have been validated and demonstrated to be very effective for the design of OTA-C filters and current conveyor-based filters. A complicated nth-order transfer function is manipulated and decomposed by a succession of innovative algebraic operations until a set of simple and feasible equations are produced. The complete filter structure is constructed by superposing the sub-circuitries realized from these simple equations. In fact, the recent ASMs can be used in the design of any kind of a linear system with a stable transfer function.
All the past filter structures enjoy the following three important criteria:                filters use grounded capacitors, and thus can absorb equivalent shunt capacitive parasitics;        filters employ only single-ended-input OTAs, thus overcoming the feed-through effects due to finite input parasitic capacitances associated with differential-input OTAs; and        filters have the least number of active and passive elements for a given order, thus reducing power consumption, chip areas, and noise.        
It has been shown that the voltage-mode filter structure with arbitrary functions needs 2n+2, i.e., n more OTAs than the other voltage-mode filter structure with only low-pass (LP), band-pass (BP), and high-pass (HP) functions. This led to the research work of a new ASM for realizing the voltage-mode high-order OTA-C all-pass (AP) and band-reject (BR) filters using only n+2 single-ended-input OTAs and n grounded capacitors.
On the other hand, combining both the current-mode notch and inverting LP signals, a current-mode HP signal can be obtained. Similarly, a current-mode AP signal can be obtained by connecting current-mode notch and inverting BP signals. This well-known concept has been demonstrated in the recently reported current-mode OTA-C universal filter structure. However, the voltage-mode circuit lacks this ability, unlike the current-mode circuit, of the arithmetic operations of direct addition or subtraction of signals. Hence, although several voltage-mode OTA-C biquad filters have been presented recently, only two of them, using three (or four) differential-input OTAs and two (or three) single-ended-input OTAs in addition to two grounded capacitors, can synthesize all the five different generic filtering signals, i.e., LP, BP, HP, BR (or notch), and AP signals, simultaneously. Therefore, the problem as to how to bring about the arithmetic superiority of the current-mode circuit to the voltage-mode counterpart and still achieve the above three important criteria for the design of OTA-C filters is an important one. Such a problem has been solved for the biquad structure with the additional valuable advantage of “programmability” using the recently reported ASM.
Although both the voltage-mode nth-order OTA-C LP, BP, and HP filter structure and the voltage-mode nth-order OTA-C AP and BR filter structure use the least number of active and passive components, namely, n+2 single-ended-input OTAs and n grounded capacitors, none of the voltage-mode nth-order OTA-C universal filter structures employs a reduced number of active and passive components. Although the voltage-mode second-order OTA-C universal filter structure is “programmable” and uses 2+2(=4) single-ended-input OTAs and 2 grounded capacitors, none of the voltage-mode nth-order OTA-C universal filter structures are “programmable”. Therefore, there does not exist any voltage-mode nth-order OTA-C universal filter structure in the published literature that has both the least number of components and the advantage of “programmability”. With these two properties in mind, a new voltage-mode nth-order programmable, universal filter structure using n+2 single-ended-input OTAs and n grounded capacitors is developed. This is an extension of the recently reported voltage-mode second-order OTA-C programmable, universal filter structure. Its fully-differential-input OTA based one can be easily obtained from the single-ended-input OTA and grounded capacitor structure using the well-known transformation method.
A differential (or double) input OTA can be realized by two parallel single-ended-input OTAs. It may be possible to synthesize an nth-order filter structure using n differential-input OTAs instead of n+2 single-ended-input OTAs in addition to n capacitors. If it is possible to do so, the following question is quite interesting: Which one is the better? Is the one with n+2 single-ended-input OTAs or the one with n differential-input OTAs? The former uses more OTAs with more non-ideal transconductance functions, but has lower parasitics for each single-ended-input OTA. The latter uses fewer OTAs with less non-ideal transconductance functions, but has larger parasitics for each differential-input OTA. Therefore, it is really worthwhile to do such a comparison between the above mentioned two cases. We then present the second new ASM to realize a voltage-mode n-th order OTA-C universal filter structure using only n differential-input OTAs and only n floating/grounded capacitors, the minimum number of active and passive components. Moreover, since a differential-input OTA can be equivalent to two parallel and complementary single-ended-input OTAs, the differential-input one can be transformed to a new complementary single-ended-input OTA based universal filter structure, which is validated to have the most precise output signals amongst the four distinct kinds of synthesized universal filter structures: (i) single-ended-in-out OTA based one, (ii) fully differential-input OTA based one, (iii) differential-input OTA based one, and (iv) complementary single-ended-input OTA based one, and two recently reported biquad filters.
In addition to output precision, the power consumption, noise, dynamic and linear ranges of the proposed four new OTA-C filter structures and some applications use H-Spice simulations. The new complementary single-ended-input OTA based one is validated to enjoy the largest dynamic and linear ranges.
As to sensitivities, second-order and sixth-order filter structures are investigated using H-Spice simulations. The realized band-pass, band-reject, and all-pass (except the fully differential one) biquads enjoy very low sensitivities achieved by the well-known passive LC ladder network. Both a direct sixth-order universal filter structure and its equivalent three-biquad-stage one are also simulated. Although some three-biquad-stage filters have lower sensitivity than their direct sixth-order one, yet some direct sixth-order filters have lower sensitivity than their equivalent three-biquad-stage ones.
An output distortion with a sudden drop in the synthesized high-pass, band-reject, and all-pass amplitude-frequency responses is investigated. A very sharp increment of the transconductance of an OTA is discovered using H-Spice simulation when the operating frequency is over a critical value. The frequency dependent transconductance function is then modified by adding an exponential-like function.
Other background information about the present invention could be found in U.S. patent application Ser. No. 11/419,313, U.S. patent application Ser. No. 12/493,184, U.S. patent application Ser. No. 12/535,194, and U.S. patent application Ser. No. 12/759,682.
To solve those negative results shown in the prior art, the present invention provides novel complementary single-ended-input OTA based filter structures. The details are described as follows.